The reason why they do it overall is less to do with technology in and of itself. By the way, they can ride in cars too. This seems paradoxical, but they don't need to drive and they already carve out exceptions for some things.
Practically speaking, it's about sovereignty. Not outsourcing "everything but your core competency" and becoming too dependent on others, just to be setup for a rugpull. Like the Hasidics, Muslims, Japanese, and Chinese, Amish are very adept at using social firewalls (even using things like language as a barrier) to keep mental viruses and bad actors out of their social networks. The aforementioned groups also very savvy at business. In these ways they are further ahead in social technologies, and the West at large has regressed.
They have even considered how they would adapt if cash was finally removed and there were a digital-only currency. They are aware of these things and their very religion was created as a "fork" out of necessity.
There's some valuable lessons in there, as walled-garden internet intensifies and every week more and more HN entrepreneurs get locked out of their frozen PayPal, Stripe, Gmail, etc and [beg for help / shame the company] on the front page as the only means of customer service. And as US has its supply chain threatened by increased tensions with their biggest supplier.